Danger – keep out
Safety-system control In a basic machine-safety system, detectors such as door switches and light curtains are typically connected to the inputs of a reliable safety relay such as the Omron G7S. The relay responds to the switch or sensor signal by signaling a contactor to turn off the applicable motor drive in order to stop the machine. Figure 2 illustrates a simple safety system built around a safety relay. A critical difference between a safety relay and a standard relay is that the safety relay has forcibly guided contacts. These ensure enhanced reliability, and allow abnormal relay operation to be detected in the event that one set of contacts becomes stuck, for example due to welding. If a normally-open (NO) contact becomes welded while the relay is energized, normally-closed (NC) contacts on the same relay
opened. A number of built-in features prevent malfunctions or incorrect operation, such as a special operation key for activating the switch. This key is designed exclusively for use with the D4BS and will not work with any other switch type. In addition, a built-in shearing-force mechanism ensures reliable separation of the contacts as soon as the switch is actuated by forcibly pulling apart the NC contacts. This ensures that any abnormalities such as contact welding will not prevent correct operation of the switch. As far as intrusion detection is concerned, light curtains are commonly used to protect operators by causing the machine to stop. A combination of transmitters and receivers set up an array of light beams, and generate an alert if one or more of the beams are interrupted. The Panasonic SF4D series of light curtains has three variants comprising the SF4D-F, which is optimized for detecting intrusion by human fingers, the SF4D-H for hand detection, and the SF4D-A for arm/foot detection. The SF4D-F can detect objects as small as 14 mm in diameter, which gives designers extra flexibility to use light curtains in compact machines in accordance with the standard ISO13855 governing the positioning of protective equipment. Other important selection criteria for light curtains include the protected height and protected width. Light curtains such as the SF4D series,
will not close when the coil is de-energized. Conversely, if an NC contact becomes welded, NO contacts on the same relay will not close when the relay is energized. The ISO 13849-1 functional safety standard requires that safety relays should comply with EN 50205. EN 50205-compliant relays such as the G7S are designed to ensure an air gap of at least 5 mm between open contacts. If the safety system requires a large number of safety relays, to handle inputs from multiple sensors, a safety PLC may be needed. Alternatively, a software- configurable safety module such as the Phoenix Contact 2986012 can allow a compact and economical solution. The 2986012 has twenty safe inputs and four safe outputs, as well as additional alarm, clock and ground-switching outputs. Using a configurable safety module, it is possible to monitor several
workpiece is passing. If non-muted light beams are interrupted the system assumes an intrusion has occurred. Some types of light curtain may require a separate controller to interpret signals from the proximity sensors. The SF4D series has muting control built in, which allows proximity sensors to be connected directly to the light curtain with no need for an external muting controller. An inductive proximity sensor such as the Panasonic GX-F12B can be used for muting control. It provides the benefit of a long sensing distance and a stable temperature characteristic that ensures consistent performance independent of the time of day or the season.
which features a small distance from the edge of the assembly to the center of the first beam, can be connected together to extend the protected area without introducing dead zones. When designing a light curtain into machinery, muting capability may be needed to allow the machine to operate normally without stopping when a workpiece enters the machine, for example on a conveyor. This ensures the light curtain will only generate a stop signal when human intrusion is detected. Proximity sensors placed near the conveyor, as shown in Figure 1, enable the system to detect the presence of a workpiece and mute the associated light beams for the duration the
Figure 3: Adding safe I/O modules to extend the safety-monitoring system.
safety functions with a single device. The monitoring system can also be extended if required by adding extra safe I/O modules, as shown in Figure 3.
Conclusion
As well as protecting the health and safety of operators, complying with machine-safety standards can help equipment producers win the confidence of customers and strengthen their position in the marketplace. An effective yet relatively simple safety system can be built around one or a small number of safety relays, which provide a highly reliable mechanism for stopping the machine in the event of a safety breach. A variety of mechanical and optical sensors are available, such as open/close switches, proximity sensors and light curtains, to inform the relay instantaneously when safety is put at risk. If extra inputs are needed, calling for a large number of safety relays, a software-configurable safety controller can provide a cost-effective alternative.
Figure 2: Simple emergency stop system using a single safety relay.
Figure 1: Proximity sensors help discriminate between a passing workpiece and intrusion by a human operator.
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